1. Field of the Invention
The present invention relates to the field of engine control and acquisition of data synchronous with the engine crankshaft, and more particularly the invention relates to the field of angular prediction methods allowing determination of the precise geometric position of the crankshaft and has application to engine control in engine manufacturers' research laboratories to design of automotive engine control systems and can also be integrated into engine control systems in a vehicle.
2. Description of the Prior Art
During the operating cycle of an internal-combustion engine, many actions can be synchronized with the geometric position of the crankshaft. This is the case for fuel injection control, spark plug control and distribution devices management. Similarly, during the research phase, engineers need to acquire signals in precise angular windows and to measure the instantaneous speed of the engine. For example, it is necessary to know the angular position of the crankshaft and its instantaneous speed within the context of control systems allowing optimizing the operating point of an internal-combustion engine through real-time processing of significant operating parameters such as the pressure prevailing in the various combustion chambers at a series of successive times of each combustion cycle.
To carry out these various actions, an engine is equipped with a calculator system that must have precise information on the crankshaft position. To meet these requirements, the crankshaft is equipped with a toothed wheel and with a sensor that detects the passage of the teeth in order to inform the calculator system managing the various control devices. This toothed wheel is referred to as target wheel.
The target wheel has become a standard. Although it has various characteristics according to manufacturers, its principle remains substantially the same: a target wheel is a toothed wheel secured to the crankshaft, whose teeth distribution and number of teeth are given. An example of a target wheel commonly encountered in Europe is a toothed wheel with 60 teeth evenly spaced out every 6°.
However, to use a target wheel, a tooth numbered 1 having a perfectly known position has to be positioned, that is the precise time when a particular tooth (tooth 1) goes past the sensor has to be determined by means of the signal from the sensor. Thus, to detect the angular position of the crankshaft, the toothed wheel generally comprises 1 or 2 missing teeth that create an absolute reference frame for the position of the crankshaft. In the aforementioned example of a target wheel with 60 consecutive teeth evenly spaced out by 6°, two consecutive teeth are missing. A “58X” type target wheel means a target wheel with “60 teeth minus 2”. Detection of the missing teeth provides an absolute reference, thus indicating the precise position of the crankshaft. By definition, tooth 1 is set as the tooth that follows the two missing teeth. FIG. 1 shows the configuration of a “58X” type target wheel denoted by CM.
Target wheels are associated with a sensor intended to detect passage of the teeth. FIG. 1 shows the signal (SB) delivered by such a sensor in the instance of a 58X target wheel. This analog signal has to be conditioned so as to be useful. The result of this conditioning (SC) is shown in FIG. 1. A rising front of signal SC is the reflection of the middle of a tooth. Detection of this rising front is precisely used as a basis by calculator systems for synchronizing engine operation. The first rising front that follows the missing teeth thus indicates the middle of the first tooth (tooth number 1) of the target wheel. The second front naturally corresponds to the second tooth and so on up to the 58th tooth. Upon passage of the missing teeth, the sensor is no longer excited until the arrival of tooth 1, which means that, for the duration of the gap, i.e. 18°, the information on the position of the teeth is no longer delivered. What is referred to as the “duration of tooth X” is the time elapsed between the passage past the sensor of tooth x and the passage past the sensor of the next tooth (denoted by x+1). This information break causes several problems for the operating system of this target wheel. In fact, the target wheel allows sequencing of the engine operation with various purposes:                teeth detection allows the operating system to be informed of the geometric position of the engine crankshaft at regular intervals. An 18° information break at the level of the missing teeth can therefore not be eliminated;        measurement of the consecutiveness of the teeth provides essential information to the operating system: the instantaneous speed of the crankshaft every 6°. The operating system can therefore not be left with a break in the measurement of the instant speed over 18° at the level of the missing teeth. FIG. 2 illustrates a measurement of the engine speed (RM) as a function of the crankshaft rotation (RV). Without any correction, it can be observed that the instantaneous speed of the engine is 1500 rpm, except at the level of the missing teeth where the duration of the last (58th) tooth corresponds to the sum of the 58th tooth, of the 59th tooth and of the 60th tooth, which amounts to dividing the engine speed by three (500 rpm);        finally, in order to optimize the number of sensors, detection of the missing teeth allows determination of the superscript suffix of the teeth and thus to locate the geometric position of the crankshaft in the revolution. As a complement to information coming from an instrumented sensor on the camshaft, precise knowledge of the geometric position of the crankshaft allows precise positioning of the injection and/or ignition windows for each cylinder.        
Thus, although the missing teeth are necessary to identify the first tooth, they cause a break in the arrival of information that sequences engine events such as injection. However, this information is essential for providing further information to the control system every 6°. For example, the control system has to be informed of the instantaneous speed corresponding to a 6°, and not 18°, crankshaft variation, to ensure coherence in the engine speed measurement.
This implies that the position of the two missing teeth (tooth 59 and tooth 60) has to be estimated in order to continue to sequence the control software and to provide coherent measurement of the instantaneous speed. Estimating the position of a tooth means completing the signal coming from the sensor and conditioned (SC) as if the wheel had no missing tooth. The duration of each tooth therefore has to be estimated: the time elapsed between the passage of tooth 58 and tooth 59 (if there was one) past the sensor, and time elapsed between the passage of tooth 59 (estimated) and tooth 60 (if there was one) past the sensor.
Estimation of the position of the missing teeth, that is of the time of passage of the missing teeth past the sensor, conventionally uses a simple interpolation or a rule of three, according to the information on the previous teeth provided by the signal delivered by the sensor and conditioned (SC).
However, this type of estimation is not acceptable for precise engine control and notably for precise determination of the injection periods according to the crankshaft position. FIG. 3 shows the engine speed (RM) as a function of the crankshaft rotation (RV). FIG. 3 is an enlargement of FIG. 2. It can be seen that the engine speed undergoes speed variations referred to as cyclic irregularities. These cyclic irregularities are linked with the various operating phases of the engine and in particular, on the one hand, the compression of the fuel mixture by the piston (crankshaft slowing down) and the explosion of the fuel mixture (crankshaft acceleration) and, on the other hand, the number of engine cylinders whose times of firing are evenly distributed over the engine combustion cycle (example: every 180° for a 4-cylinder 4-stroke engine, every 120° for a 6-cylinder 4-stroke engine). This involves, on the one hand, that the duration of the missing teeth varies from one crankshaft revolution to the next and, on the other hand, that the durations of teeth 58, 59 and 60 are different and follow a variation depending on the cyclic irregularities.